Lab-on-a-chip (LOC) based device requirements for analyte detection are sensitivity, universality and portability. To this date, these conditions have not been fully met and detection remains the main challenge in the development of LOC technology. Optical detectors, including fluorescence detection, have demonstrated the highest sensitivity. However, optical detectors are not universal and not easily made portable due to the size of the light sources. The use of electrochemical methods is well-suited for integration into portable systems, but they are less sensitive and prone to interferences. From the group of electrochemical sensors, C4D detectors are the most appealing as they fulfill the requirements of portability, universality for charged analytes and acceptable sensitivity.
The principle of C4D in combination with electrophoresis will now be described with reference to FIG. 1. FIG. 1 shows an arrangement of two external metal electrodes 100a, 100b in close proximity to an electrophoretic separation channel 102 in a microfluidic chip 104. The microfluidic chip 104 comprises two polymer sheets, namely top sheet 104a and bottom sheet 104b. The top sheet 104a provides access to reservoirs as will be described below, and the bottom sheet 104b provides the separation channel 102 that has been hot embossed into the bottom sheet 104b. In use, a run buffer reservoir 107, a first sample reservoir 109 and an outlet reservoir 111 of the microfluidic chip 104 are filled with electrophoretic run buffer solution, and a second sample reservoir 113 is filled with target analytes, typically ionic species dissolved in the run buffer solution. A separation voltage is then applied between the second sample reservoir 113 and the first sample reservoir 109. This drives ‘plugs’ of ions 114 into the separation channel 102. Subsequently, the separation voltage is applied between the run buffer reservoir 107 and the outlet reservoir 111 with all other reservoirs floating. This causes the plugs of ions 114 to be driven towards the electrodes 100a, 100b for detection.
The two external metal electrodes 100a, 100b and the electrophoretic separation channel 102 together form the C4D cell or detection cell. When the upstream/emitting electrode 100a emits an AC signal through the channel 102, it is capacitively captured by the downstream/receiving electrode 100b. The electrodes 100a, 100b are in the same plane and are attached to a top plate that seals the channel 102 and are typically placed in an anti-parallel configuration with respect to the length of the channel 102. The applied AC signal (typically 50-600 kHz) from the emitting electrode 100a capacitively couples through the channel 102 to the receiving electrode 100b, resulting in a small current that is amplified by an amplifier 106, rectified and offset-corrected using a rectifier 108, filtered and that undergoes data acquisition using a data acquisition tool (DAQ) 110 and finally recorded in a computing device 112 or other storage device as a DAQ graph.
The C4D cell can be considered as a combination capacitor-resistor-capacitor (CRC) electrical circuit, where the electrodes 100a, 100b and the channel's electric double layer form the capacitors, and the section of the channel 102 between the electrodes 100a, 100b forms the resistor. When a plug of ions 114 is driven through the section of the channel between the electrodes, the measured impedance of the system changes instantaneously because of change in the resistance due to the different conductivity of ionic species passing through the electrodes with the background electrolyte. In practical terms, this leads to a sudden change in the zero leveled output voltage or a peak in the DAQ graph. By electrophoresis, separated plugs of ions can be driven through the C4D cell at different times and the corresponding signal recorded, thus obtaining separated peaks according to the times at which the ions cross the C4D cell. Each peak is related by time to a specific ion, and the area under the peaks is related to the concentration of the specific ion. C4D in combination with electrophoresis therefore provides qualitative and quantitative analysis.
The C4D cells reported to date use two electrodes placed externally over the separation channel. An example is illustrated in FIGS. 2(a) and (b), which respectively show a perspective view and plan view of a conventional detection cell. As noted earlier, electrodes 100a, 100b in conventional detectors are fixed to a top plate 200 that seals the separation channel 102 and are typically placed in an anti-parallel configuration with respect to the channel 102. In this configuration, the capacitance coupling to the solution in the channel 102 is inefficient and requires a high frequency and high voltage to couple the signal to detect low concentration samples. High frequencies, however, result in stray capacitance having a more significant effect. Changes in the conductivity of the solution will then only result in a small change over the background signal.
To reduce or eliminate the stray capacitance, different strategies have been employed such as placing a ground plane 202 between the electrodes 100a, 100b to shield their direct crosstalk (as shown in FIGS. 2(a) and (b)). However, while these strategies decrease the stray capacitance somewhat, the resulting detection sensitivity remains limited.
One alternative option to improve capacitance is to increase the magnitude of the AC voltage. However, high voltage levels are difficult to produce and are not safe to handle in portable systems. Another option to have increased capacitance is to use relatively large electrodes or detection lengths, but these approaches severely decrease resolution.
Without compromising resolution, one effective way to increase sensitivity in capacitive coupling detection is to reduce the distance between the electrodes and the detection area or the section of the channel between the electrodes (also known as the ‘detection cell volume’). Known arrangements have achieved this by either: (i) scribing off some portion from the chip surface so that electrodes can be disposed nearer to the channel, or (ii) incorporating electrodes within the chip (integrated chip) during the microfabrication so that they are close enough to the channel. These approaches are either inaccurate (for option (i) above) or require complex fabrication processes (for option (ii) above).